When a "C" dolly is used in conjunction with the standard "A" train vehicle configuration it creates a new configuration called a "C" train which has one less point of articulation in the vertical plane than an "A" train. As a result, this is a much more stable and controllable configuration.
As a result of testing, it has been demonstrated that the amount of resistance to steer due to side forces on a dolly of this type is extremely critical at high speeds. When the vehicle is in an evasive maneuver, a dolly with a relatively small amount of resistance to steer, due to side forces, will contribute to high speed off tracking of the rear vehicle (possibly intruding into a lane of oncoming traffic). The higher the resistance to steer, the better are the high speed off-tracking characteristics of the complete "C" train configuration. Ideally, the dolly steering should be locked at high speeds so that it becomes a fixed axle. Steering is not required at higher speeds since speeds over approximately 30 mph (50 kph) do not normally include relatively sharp or violent turning maneuvers. These usually occur at relatively low speeds.
The industry has attempted to develop a dolly with a resistance to steer of approximately 0.3 G. This means that if there is approximately 20,000 pounds of load on the tires at ground level, the axle could not steer more than one degree off centre in either direction when a force equivalent to approximately 6,000 pounds was applied to the tires perpendicular to the longitudinal axis of the dolly and "C" train configuration. Although this criterion greatly improves the high speed off-tracking characteristics of the "C" train unit, it also results in a high resistance to steer at low speeds (for example below 30 mph) which creates severe and undue stresses and forces in the dolly and its structure, particularly during tight turning maneuvers. It has been noted that in such situations, the forces are so high that the tires are nearly being forced off the rims of the "C" dolly.